Better Assert (with error message) for timestamp range

[lmolkova]

GoSpanData_EndedSpan test became flaky after timestamp changes, this PR only improves assert to generate error message and does nothing to fix the test

Expected: True
Actual: False

Stack trace
   at OpenTelemetry.Trace.Test.SpanTest.AssertApproxSameTimestamp(DateTime one, DateTime two) in D:\a\1\s\test\OpenTelemetry.Tests\Impl\Trace\SpanTest.cs:line 703
   at OpenTelemetry.Trace.Test.SpanTest.<GoSpanData_EndedSpan>d__11.MoveNext() in D:\a\1\s\test\OpenTelemetry.Tests\Impl\Trace\SpanTest.cs:line 303
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

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